Attention is directed to copending applications Ser. No. 10/000,345 filed herewith, entitled, xe2x80x9cControlling Gloss in an Offset Ink Jet Printerxe2x80x9d and Ser. No. 10/000,336 filed herewith, entitled, xe2x80x9cContinuous Transfer and Fusing Application System.xe2x80x9d The disclosure of these references is hereby incorporated by reference in their entirety.
The present invention relates generally to a drum for fixing an ink image on a receiving medium and, more particularly, to a multi-layer drum that creates a narrow, high pressure nip and includes an outer compliant elastomeric layer that provides improved ink image fixation on the receiving medium with reduced thermal requirements.
For printing in a solid-ink printer, a common method of applying droplets of ink onto a piece of paper is to directly print the image onto the paper, i.e., a process known as direct printing. However, direct printing has many disadvantages. First, the head to paper gap must be adjusted for different media in order to control drop position. Second, there is the well-known paper hand-off problem between the rollers that guide the paper, because of the large size of the head. Third, there is a concern that head reliability will decrease because the paper is near the head. These problems are addressed with an offset process. In this process, the ink is first applied to a rotating drum and is then transferred off the drum and fixed into the paper in a single pass. This process is known as a transfix process or a transfuse process. Therefore, a single drum surface transfers the image, spreads the pixels, penetrates the pixels into the media, and controls the topography of the ink to increase paper gloss and transparency haze. The process requires a delicate balance of drum temperature, paper temperature, transfix load, and drum and transfix roller materials and properties in order to achieve image quality. These combined requirements reduce the drum material possibilities mainly due to wear of weaker materials, which result in gloss and haze degradation.
Ink jet printing systems utilizing intermediate transfer ink jet recording methods, such as that disclosed in U.S. Pat. No. 5,389,958 entitled IMAGING PROCESS and assigned to the assignee of the present application (the ""958 patent) is an example of an indirect or offset printing architecture that utilizes phase change ink. A release agent application defining an intermediate transfer surface is applied by a wicking pad that is housed within an applicator apparatus. Prior to imaging, the applicator is raised into contact with the rotating drum to apply or replenish the liquid intermediate transfer surface.
Once the liquid intermediate transfer surface has been applied, the applicator is retracted and the print head ejects drops of ink to form the ink image on the liquid intermediate transfer surface. The ink is applied in molten form, having been melted from its solid state form. The ink image solidifies on the liquid intermediate transfer surface by cooling to a malleable solid intermediate state as the drum continues to rotate. When the imaging has been completed, a transfer roller is moved into contact with the drum to form a pressurized transfer nip between the roller and the curved surface of the intermediate transfer surface/drum. A final receiving substrate, such as a sheet of media, is then fed into the transfer nip and the ink image is transferred to the final receiving substrate.
In this standard offset process, the release agent application must be applied every print. This provides a release layer that facilitates image transfer. Therefore, unlike a typical laser printer process in which the deposition of the toner onto the paper and the fusing of the paper occurs in parallel (at the same time), the current solid-ink process operates in series. Therefore, to increase print speed, this architecture requires very high transfix velocities and release agent application. High transfix velocities are not very compatible with the current transfix process because of the combined paper preheat and duplex requirements (as the transfix velocity increases, the paper preheater temperature must increase to achieve the same exit paper temperature and if the preheat temperature is over about 60-65 degree C. the duplex image will smear). However, even in the fastest of possible speeds, this serial process drastically decreases the print speed. Higher loads can be used to offset some of the losses due to high transfix velocities, however, even now the required loads with this process are very high (currently about 800 lbs).
Additionally, it is known that higher drum temperature is better for drop spread and image durability. However, in current systems the drum temperature is limited by the cohesive failure of the ink. Cohesive failure results from the ink layer fracturing as the ink and paper leave the nip instead of the oil layer splitting which would normally allow complete transfer of the ink off the drum and onto the paper. Due to the large thermal mass of the imaging drum and the relatively short time required to transfix an image, there is no time for heating or quenching in a transfix nip. Therefore, the transfix temperature in these systems is limited by the cohesive failure of the ink.
To provide acceptable image transfer and final image quality, an appropriate combination of pressure and temperature must be applied to the ink image on the final receiving substrate. U.S. Pat. No. 6,196,675 entitled APPARATUS AND METHOD FOR IMAGE FUSING and assigned to the assignee of the present application (the ""675 patent) discloses a roller for fixing an ink image on a final receiving substrate. The preferred embodiment of the roller is described in the context of an offset ink jet printing apparatus similar to the one described in the ""958 patent. In this embodiment, an apparatus and related method for improved image fusing in an ink jet printing system are provided. An ink image is transferred to a final receiving substrate by passing the substrate through a transfer nip. The substrate and ink image are then passed through a fusing nip that fuses the ink image into the final receiving substrate. Utilizing separate image transfer and image fusing operations allows improved image fusing and faster print speeds. The secondary fusing operation enables the image transfer process to use reduced pressures, whereby the load on the drum and transfer roller is reduced. Therefore what is needed is a transfer surface application system that overcomes the drawbacks of previous application systems using separate transfer and fusing operations.
It is an object of the present invention to provide an improved imaging apparatus and method for a compliant surface for near perfect image transfer and a secondary fuser that is capable of operating at a temperature more independent of the cohesive failure limits.
It is yet another objective of the present invention to provide an improved apparatus and method for applying a compliant surface that increase the reliability of the printer, decreases the noise and decreases the cost of the release agent system.
Accordingly, the present invention is a compliant imaging surface for offset printing comprising a drum having an outer compliant elastomeric layer affixed to the drum. The outer compliant elastomeric layer is sufficiently compliant to contact ink pixels having at least first and second heights so as to fix the ink pixels to the final receiving medium. A second outer rigid layer is affixed to the outer compliant elastomeric layer wherein the second outer rigid layer is thinner than the outer compliant elastomeric layer and acts to keep pressure across a span of one pixel with the outer compliant elastomeric layer acting to allow deformation on the pixel to pixel span.
Still other aspects of the present invention will become apparent to those skilled in this art from the following description, wherein there is shown and described a preferred embodiment of this invention by way of illustration of one of the modes best suited to carry out the invention. The invention is capable of other different embodiments and its details are. capable of modifications in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.